We report here that CD40- but not lipopolysaccharide (LPS)-activated murine dendritic cells (DC) express OX40-ligand (OX40L) as has been reported in humans. To understand how OX40 ligation affects differentiation of CD4 T cells at the time of priming, we constitutively expressed OX40L on DC using the DC-specific promoter of CD11c. Transgenic mice showed greatly increased numbers of CD4 but not CD8 T cells in their B cell areas. This effect was to a great extent immunization dependent, as spleen and lymphoid tissue with no germinal center reactions from mice which had not been deliberately immunized did not show marked CD4 T cell accumulation. The increased numbers of CD4+ CD62low cells in transgenic mice suggest that it is activated CD4 T cells that accumulate within B cell follicles. These data are consistent with the notion that physiological engagement of OX40 (CD134) on activated CD4 T cells either initiates their migration into or causes them to be retained in B follicles. In contrast, LPS-treated CD did not up-regulate OX40L expression. This dichotomy provides a molecular explanation of how DC might integrate environmental and accessory signals to control cytokine differentiation and migration in CD4 effector cells.